This invention relates to ultrasonic transducer systems which have an ultrasound transducer supported by a filter.
U.S. Pat. No. 5,275,060 discloses such an ultrasonic transducer system. Ultrasound transducers can be used in gas meters, for example. Pairs of ultrasound transducers define a measuring path which lies at an angle other than a right angle to the longitudinal axis of the fluid flow. Measurements make use of the difference in transition times between two ultrasonic signals which have a component in the flow direction and another component against the flow direction. The flow velocity can be calculated from the measured difference in the transition time.
The piezo crystal which generates ultrasound waves in known ultrasonic transducer systems is mounted to the measuring instrument via an acoustic filter. The purpose for the acoustic filter is to suppress inductive disturbances, or cross-talk, generated by transmissions between an ultrasonic emitter and an ultrasonic receiver. Such filters have a cylindrical shape and a multitude of alternatingly arranged sections of greater and lesser wall thickness to provide segments of higher and lower impedance relative to axially translating motions. This arrangement at least reduces the effect of cross-talk through the housing on the ultrasonic signals.
Since ultrasonic signals transmitted through the housing have significantly shorter transit times than signals which propagate through the fluid medium to be measured, cross-talk or inductive disturbances can be a source of significant interference.
However, this known acoustic filter is primarily effective for dampening axial forces. Ultrasound transducers attached to circular plates for uncoupling ultrasound transmissions through solid bodies primarily subject the filter to torsional moments which, amongst others, lead to radial strain waves and radial oscillations in the components, which subjects the ultrasonic signals to cross-talk. The known acoustic filter of U.S. Pat. No. 5,275,060 provides insufficient dampening of such waves.
In view of this state of the art, it is an object of the invention to provide an improved ultrasonic transducer system which significantly improves the suppression of cross-talk through the housing, and particularly cross-talk caused by radial strain waves and oscillations.
According to the invention, the filter of the ultrasonic transducer system is coupled to a deflection or vibration plate of the ultrasound transducer from which ultrasound waves radiate. The filter has a deflecting segment which transforms a radial deflection of a first section of the filter into a rotational or torsional deflection of a second section of the filter. The arrangement provided by the present invention has the particular advantage that by combining radial vibrations of the first section and torsional vibrations of the second section via the deflection plate, an optimal filtering of the ultrasonic signals is attained. This effectively eliminates or at least greatly reduces the transmission of cross-talk signals from the ultrasound transducer into the housing of the measuring instrument.
The deflecting segment converts radial deflections of the first section into torsional deflections of the second section. By appropriately dimensioning the subsystem consisting of the two sections and the deflecting segment, a virtually motionless state of the second section is attained over a frequency range that includes the operational frequency range of the ultrasound transducer. Thus, the second section is subjected to substantially no torsional vibrations. An optimal filtration effect is attained in this manner.
The filter is preferably coupled to the rim of a pliant vibration plate. The ultrasound generating component causes the deflection of a mid-portion of the bending plate. In this manner, the filter is subjected to only small axial forces. The vibration plate and the first section of the filter are preferably connected by soldering or welding the first section directly to the rim of the vibration plate.
The ultrasonic transducer system of the invention is preferably of a simple, rotationally symmetric construction.
In a similarly simple manner, the deflecting segment preferably connects the longitudinal end of the first section with an inner side of the second section so that radial movements of the longitudinal end of the first section are transformed into torsional movements of the second section.
The deflecting section is conveniently and preferably a simple disc which has a 90xc2x0 bend.
In an easily manufactured embodiment of the invention, the first section is formed as a tubular casing with a wall thickness that preferably changes in the longitudinal direction and which preferably increases in the direction of the second section. This suppresses resonances caused by the thickness of the tubular casing, which can be the source of cross-talk and reduce the available band width of the filter.
The second section is preferably manufactured as a torsion ring.
To facilitate the radiation of ultrasound waves from the mid-portion of the vibration plate while minimizing the transmission of axial forces to the filter, the vibration plate preferably has a mass ring at its periphery.
For attaching the ultrasonic transducer system to a housing of a measuring system, an attachment flange is provided which secures the filter and therewith the ultrasound transducer.
It is particularly advantageous when the filter with its two sections and the deflecting segment are of a one-piece construction by assembling the components into a single unit.
The filter and its components should be dimensioned so that no resonances are generated over the operational frequency band width of the ultrasound transducer. For example, the first torsional resonance of the torsion ring and its radial resonance should lie below the operational band width of the filter.
An optimal effectiveness of the filter results from dimensioning the components so that the second section is preferably free of vibrations over the operational frequency band due to its rotational mass so that essentially no vibrations are transferred to the housings of the measuring system.
The length of the first section is selected so that axial resonances are outside the operational frequency band width.
Exemplary embodiments of the invention are described with reference to the drawings.